At the request of our membrane switch customers, who were searching for new ways to justify automating their metal dome placement, Memcon with Assembly Automation developed equipment for a process to successfully place LEDs and other SMD components to printed circuits with conductive adhesives.
The equipment and process developed is now widely used throughout the world.
From the first it was understood that using dot dispensing of adhesives for any SMD package size less than 1206 was difficult if not impossible to place. Package height translates to cost per (increased spacer height or need to emboss). Listening to our customers, Memcon developed an extremely Thin LED specifically for this market. Memcon LED is a 1206 package, which is easy to place because of its large width and length, but is as thin, 0.35 mm (0.016”) and as virtually any LED on the market. Memcon offers a wide range of colors from stock.
Based on our experience we have developed guidelines for the pad design and the dispensing of adhesives to improve bond strength and reduce shorts. The most significant process improvement is the use of center drop of non-conductive adhesive in between the pads in order to improve the bond strength of the joint and to serve as a dielectric to prevent shorting of the two conductive adhesive drops that are dispensed over the pads.
The suggested design of the pad is shown in Figure below. The biggest improvement of this pad design is the use of the 1 mm diameter circle in the center of the pad. The purpose of this circle is to create a bond directly between the conductive adhesive and the substrate, which is stronger than the bond between the printed ink and the substrate that is typically the failure point of any flexing of the joint. This technique has shown up to a 35% increase of shear bond strength.